Unicode conversion issue

[Jaegeuk Kim]

Hi Linus/Gabriel,

Once Android applied the below patch [1], some special characters started to be
converted differently resulting in different length, so that f2fs cannot find
the filename correctly which was created when the kernel didn't have [1].

There is one bug report in [2] where describes more details. In order to avoid
this, could you please consider reverting [1] asap? Or, is there any other
way to keep the conversion while addressing CVE? It's very hard for f2fs to
distinguish two valid converted lengths before/after [1].

[1] 5c26d2f1d3f5 ("unicode: Don't special case ignorable code points")
[2] https://bugzilla.kernel.org/show_bug.cgi?id=219586